Photosensitive flexographic printing plates have a layer of a photo-polymer which undergoes a chemical reaction when exposed to actinic radiation. Typically ultraviolet light is used to expose the photo-polymer in flexographic printing plates. Upon exposure to ultraviolet light the photo-polymer undergoes a cross-linking reaction. The printing plates are exposed through a mask which blocks light from reaching certain portions of the photo-polymer. The mask is patterned with a pattern of opaque and non-opaque areas which form an image to be recreated on the printing plate. After exposure the printing plate can be developed. After development the pattern of exposed and unexposed areas on the printing plate provide a textured surface which may be used to print an image.
Some types of printing plate are used in conjunction with a separate mask. Typically the mask is made from a half-tone film, such as a silver-halide film, which can be placed on top of the plate before the plate is exposed. A problem with this method is that it is necessary to keep the film in intimate contact with and in accurate alignment with the plate during exposure. If the mask is misaligned with the printing plate or if the mask is not kept in intimate contact with the printing plate during exposure then the results obtained will be less then optimum. Even when a separate mask is applied carefully to a printing plate the results obtained are often not optimal.
Another method for exposing flexographic plates involves providing a photosensitive plate which has a mask formed integrally with the plate. Such plates include a layer of a photosensitive material such as a photo-polymer gel underlying a layer of mask material. Because the mask is integral with the printing plate, this approach provides a mask which is always in intimate contact with the photo-polymer layer and which is always properly aligned with the printing plate. A problem with this method is that the material from which the mask layer is made can diffuse into the photo-polymer gel. Over time this can degrade the gel. As a result, printing plates which include an integral mask may have a low shelf life.
The problem of low shelf-life is exacerbated because there are a large number of different types of photo-polymers used in printing plates. All have different characteristics and are used in different applications. If each of the different types of printing plates has a short shelf life then it is often not practical for a user to keep on hand a stock of printing plates of any types which will not be quickly used up.
A further problem with printing plates which have an integral masking layer is that the masking layer on existing plates is typically fragile. A scratch in any portion of the masking layer can make the entire plate unusable.
U.S. Pat. No. 5,262,275 discloses a flexographic printing element which incorporates a barrier layer which separates an infrared ablatable masking layer and a photo-polymerizable layer. The barrier layer prevents migration of material from the masking layer into the photo-polymerizable layer. While this construction improves the shelf life of a printing plate it does not avoid the requirement to maintain a number of different types of printing plate in stock which have different types of photo-polymer layer. It also does not address the problem that the masking layer of a printing plate may become damaged, thereby making the plate unusable.
There is a need for a method for imaging flexographic printing plates which avoids or reduces at least some of the difficulties which are inherent in the above-noted existing technologies.